Secondary Dominants

Every chord in the preceding eight units could be built from within a single scale — white keys only in C major, the three flats of the signature in C minor.

From here the notes come from outside the scale, and the first step out is the secondary dominant. When a score shows accidentals the key signature never declared, this is what they are a substantial fraction of the time.

Relocating the tonic, temporarily

One idea explains the whole mechanism.

Pick one diatonic chord and decide, for a moment, that it is the tonic.

If there is a tonic there is also a dominant. Placing that dominant in front is what a secondary dominant does. To treat ii, Dm, as a temporary tonic in C major, note that the dominant of D is A, and put A7 before the Dm.

The notation is V7/x. What follows the slash is the temporary tonic, so V7/V means “the V of V” — the dominant seventh of V treated as a tonic. The slash has nothing to do with slash chords; read it as “of.”

Building one takes two steps.

1. take the note a perfect 5th above the root of the target. That is the root of the secondary dominant
2. build a dominant seventh (major triad plus minor 7th) on it

The secondary dominants available in C major are these.

Target	Temporary tonic	Secondary dominant	Note outside the scale
ii	Dm	V7/ii = A7	C#
iii	Em	V7/iii = B7	D# and F#
IV	F	V7/IV = C7	Bb
V	G	V7/V = D7	F#
vi	Am	V7/vi = E7	G#

Notice that vii° is absent. vii° is a diminished triad, and a diminished triad cannot serve as a tonic, since it does not function as a stable destination. Because the mechanism presupposes a temporary tonic, no secondary dominant can be built for a chord that cannot be one.

Check

Building the secondary dominant of vi (Am) in C major, what is its root and what accidental is added?

Root C, accidental BbRoot E, accidental F#Root A, accidental C#Root E, accidental G#

Check answer

The altered note is the leading tone of the destination

Why do secondary dominants generate so much momentum? Because the added accidental functions as a leading tone.

Units 2 and 4 covered what a leading tone does: the seventh degree sits a half step below the tonic and is drawn to it. The altered note of a secondary dominant sits in exactly that position. The F# of V7/V is a half step below G; the G# of V7/vi is a half step below A.

A diatonic ii, and one preceded by its secondary dominant

▶Without secondary dominant▶With secondary dominant

Check: The first moves straight to Dm. The second inserts A7 in front of it. The inserted version makes the arrival on Dm feel far more decided. Only one note has been added, C#, and that one note presents Dm as a momentary tonic. Follow the way the C# gets pulled toward D.

I – ii – V – I

The part-writing rules for dominant sevenths carry over unchanged. The altered note, the temporary leading tone, moves up a half step, and the seventh moves down by step.

Following the alto in Fig. 2 gives C, C#, D — rising by half steps. Inserting a secondary dominant produces a chromatic line like this in one voice or another. That is the characteristic texture of chromatic harmony.

V7/IV turns the tonic itself into a dominant

One entry in the table is unlike the others: V7/IV. A perfect 5th above IV is the tonic itself, so the tonic chord becomes a dominant seventh.

In Fig. 3 the root does not move at all. The alto simply falls from C to Bb, and a stable tonic turns into a chord that has to go somewhere. It is the clearest available demonstration that adding one note can reverse a chord’s function.

Blues and rock putting a seventh on the tonic from the opening measure is this device become idiomatic. In that repertoire, though, the chord often stays put as a tonic rather than moving to IV, and when it does it has left the framework of secondary dominants entirely. Unit 9 takes that up as the modal tonic chord I7.

Check

How does V7/V (D7) in C major differ from the diatonic ii (Dm)?

The third changes from F to F# and a seventh, C, is addedThe fifth changesThey are the same chordThe roots differ

Check answer

Secondary dominants chain

The relationship in which a dominant seventh moves to the chord a perfect 5th below it can be linked.

E7 → Am → D7 → G7 → C. The roots descend by perfect 5ths: E, A, D, G, C. That is walking counterclockwise around the circle of fifths.

A chain of secondary dominants

▶A chain of five chords▶The chain with the intermediate chords altered

Check: The roots keep falling by perfect 5ths. Every joint is a dominant-seventh-to-tonic motion, so the moment you think you have landed, the chord becomes another dominant seventh and pushes on again. That continuous falling is the skeleton of the circle progressions in jazz and city pop. Listen for resolution being withheld right up to the final C.

E7 – Am – D7 – G7 – C

The second variant makes every intermediate chord a dominant seventh as well. No destination ever materializes before the next dominant seventh arrives, so tension accumulates and is carried through to the end.

▶ Enter C A7 Dm G7 C in the progression analyzer Non-diatonic chords such as A7 display in gray. Looking at what follows each gray chord tells you the x in V7/x Chord Progression Analyzer →

Secondary dominants are the simplest logic among the ways of using notes from outside the scale. Choose a target, build its dominant seventh. That is all there is to it.

The next lesson takes a different route in: rather than treating a chord as a temporary tonic, it borrows chords from the other mode on the same tonic.